Conventional carbonaceous furnaces for the processing of steel, etc. have included both electric and gas heating elements. Early furnace designs included a plurality of relatively small heating elements in spaced relation in a furnace heated by the flow of electric current therethrough. Such elements require a relatively high voltage, which results in electric insulation problems and current leakage through the refractory lining of the furnace due to carbon deposits. High voltage is also responsible for or injury to personnel who accidentally touch the elements.
Another difficulty encountered is control of the desired radiant heat distribution in the furnace and particularly in a heat treating furnace.
In an effort to overcome the above disadvantages, relatively large flat heating elements were extended over a large area of the furnace wall to produce a uniform heating effect. The heating elements were formed by corrugated sheets or strips mounted adjacent to the furnace walls. These corrugated heating elements are set forth in U.S. Pat. No. 2,896,004 (Duffy et al.), issued July 21, 1959.
The corrugated heating elements, as noted above, resulted in a maintenance problem, wherein replacement required entering the furnace and removing the elements from the furnace walls. In order to overcome this maintenance problem, U-tube heating elements were developed. A typical U-tube heating element is disclosed in U.S. Pat. No. 4,332,552 (Seelandt), which issued on June 1, 1982. The U-tube heating elements solved the maintenance problems associated with the corrugated heating elements by permitting removal of the elements through the roof of the furnace.
The U-tube heating element operates on extremely low voltage, which permitted disposition of the element directly in a carbonaceous atmosphere. Since there is no need to isolate the element from the atmosphere, the element is free to radiate directly within the heat chamber, thereby lowering element operating temperatures over those designs where the elements are placed inside of the radiant tube. The U-tube elements are plug-mounted in the roof for ease of service. Four elements are usually utilized, two per sidewall, for maximum radiating area and extend above, below and beyond the workpiece.
Unfortunately, U-tube heating elements require very large openings in the furnace roof and are not interchangeable with gas fired, "single-ended" radiant tubes; that is to say, straight tubes requiring only that a single small roof opening be provided, contrasted with U-shaped elements. Therefore, the present inventor undertook the development of a unique electrified single-ended tube heating device which is easily replaceable and can be interchanged with the aforesaid gas fired, single-ended radiant tubes.
The present invention overcomes the high voltage and maintenance problems of conventional furnace heating elements. That is, the electrified single-ended tube heating device of the present invention is a low voltage heating device which permits operating directly in a carbonaceous atmosphere. Its unique design also facilitates field replacement or conversion to gas fired single-ended radiant tubes while minimizing the size of the roof openings.
The present invention also provides many additional advantages which shall become apparent as described below.